SU9: Ard Environments SA

• The Kalahari Landscape covers >2.5 million km2 of multiple countries in southern Africa: South Africa, Namibia, Botswana, Angola, Zimbabwe, Zambia and the Democratic Republic of Congo

  
• The Kalahari Landscape is located in the northernmost Northern Cape and parts of North-West Province in South Africa
• The Kalahari Landscape has an elevation of *1,000 m.a.s.l. and receives <500 mm of rainfall per year
• Kalahari Landscape


  Covered by sandy sediment and is technically arid or semi-arid, but well vegetated with shrub or bush savanna
• The extreme south and southwest regions of the Kalahari Landscape take on a true desert-like appearance with sparse shrubs and grasses, partially vegetated dunes, seasonally inundated pan depressions, and dry valleys that occasionally flood after heavy rainfall
• The Kalahari Landscape includes the Kgalagadi Transfrontier Park which covers ~38000km2
• Kalahari Group sediments


  Late Cretaceous and Paleogene sediments deposited in the newly formed Kalahari Basin due to subsidence of the continental interior, later reworked by wind
• Development of the Kalahari Landscape

  1. Linked to the evolution of the African landmass after the break-up of the supercontinent Gondwanaland during the Mesozoic

  2. Separation of Africa from other landmasses led to heating and rifting of the crust, resulting in the uplift of the southern African continental margin in the mid- to Late Cretaceous

  3. Tectonic uplift and downwarping events during the Paleogene, Neogene, and Quaternary influenced the topography and drainage systems of the region
• The present course of the Orange River was established by the Late Oligocene, and a period of relative tectonic stability in the mid-Miocene led to silica and carbonate cementation forming thick sequences of silcrete and calcrete duricrusts
• Duricrusts


  Influence the landscape of the south-west Kalahari, forming an extensive calcrete-supported plateau surface
• Development of the Kalahari Landscape

  1. Wind reworking of the uppermost unconsolidated component of the Kalahari Group (Kalahari sands) in the Pliocene and Quaternary

  2. Additional episodes of calcrete formations in the late-Quaternary
• Dry Valleys


  • Prominent feature of the Kalahari Landscape, developed over longer timescales than the current dunes
  • Main southern Kalahari drainage systems include the north-south draining Auob and Nossob rivers, and the east-west draining Kuruman and Molopo rivers
  • Predominantly fossil Middle Kalahari systems that drained internally towards the Makgadikgadi depression or Okavango River and Delta in Botswana
  • Cross-sectional shape varies from gently sloping to steep-sided gorge-like sections, cut through the south-west Kalahari calcrete plateau
  • Some tributaries originate in areas with a Kalahari sand cover, and the valleys are dry over much of their length except for their headwater sections or during large-scale flood events
• Historic and prehistoric flood events have affected the morphology of the main southern Kalahari valleys, with abandoned channels and meander systems observed in wider sections
• Dunes and Dunefields


  • ~85% of the south-west Kalahari consists of dunefields, with the remaining ~15% being sand sheets, valleys, or pans
  • The southern margin of the dunefield is marked by the Orange River, with the dunes becoming less distinct north and east of the Nossob and Molopo valleys
  • The dunefield is vegetated, which influences the Aeolian processes, with sand movement occurring only on dune crests in areas where vegetation has been cleared or during periods of strong winds
  • The south-west Kalahari dunefield is mostly covered by orange-red parallel to sub-parallel linear dunes with rounded or sharp crests, oriented northwest to southeast, and ranging in height from 2 m to over 30 m above interdune areas
• The linear dunes in the south-west Kalahari have been shaped over millennia by the movement of fine- to medium-grained sands in a southeasterly direction towards the Orange River
• Dunes and Dunefields of the Kalahari Landscape
• Linear Dunes


  • Shaped over millennia
  • Movement of fine- to medium-grained sands
  • Southeasterly direction towards the Orange River
• Linear Dunes in South Africa

  • Extend to Kgalagadi Transfrontier Park as far south as Upington
  • Range in height from 2 m to over 30 m above interdune areas
  • Highest around Auob and Nossob
  • Up to 250 m wide, with interdune spacings of up to 2.5 km
• Types of linear dune forms in South Africa


  • Discontinuous simple linear dune forms
  • Simple continuous forms that extend for several kilometres
  • Compound forms where branching Y-junctions
  • Compound forms with more obtuse angles between branches
  • Discontinuous dunes with no preferred orientation
• Parabolic Dunes


  • U-shaped dunes with elongated arms pointing in the direction opposite to the prevailing wind
  • Formed in areas with sufficient sand and vegetation that stabilizes the dune arms
• Barchan Dunes


  • Crescent-shaped dunes with gently sloping windward sides and steep slip faces on the leeward side
  • Formed in areas with limited sand supply and consistent wind direction
  • Migrate in the direction of the prevailing wind
• Climbing and Falling Dunes


  • Exhibit periodic movement or migration due to changing wind patterns
  • Climbing dunes move uphill in the direction of the prevailing wind, while falling dunes move downhill against the wind
  • Can transition between various forms depending on wind conditions
• Flank Dunes


  • Small dunes that form on the sides of larger dunes
  • Elongated and parallel to the prevailing wind direction
  • Often occur in areas with abundant sand supply and strong winds
• Crescentic or "Lunette" Dunes


  • Occur on many pan margins
  • Act or have acted as sediment 'feeders' for linear dunes which continue downwind from them
• Valley-margin Dunes


  • Found typically at the top of valley flanks
  • Narrow, elongate form with arcuate and straight planform elements
  • Mirror the course of contemporary and ancient channels
• Luminescence dating of wind-blown deposits in the south-west Kalahari revealed a long history of aeolian activity across the Kalahari dating back to 200 ka (thousand years)
• Phases of aeolian activity occurred in the south-west Kalahari at 104, 77 and 58–52 ka
• Wind-blown sediments have been accumulating semi-continuously in the south-west Kalahari from around 35 ka to the present day
• Pans


  • Small, closed basins or depressions with seasonally flooded lakes found in arid to semi-arid regions of low relief
  • Serve as important water sources for humans and wildlife
  • Occur throughout the Kalahari, particularly in areas with annual rainfall <500 mm and susceptible underlying rock or sediment
  • Range in size from 1 to 16 km2 and depths reaching 20 m below the surrounding ground surface
  • Have sub-circular, sub-elliptical, or kidney shapes, often with a preferred orientation parallel to the prevailing wind
• Examples of pans in South Africa

  • Hakskeen Pan
  • Koppieskraal Pan
  • Polentswa Pan
  • Bitterpan
• Lunette Dunes


  • Paler in color than the surrounding sands
  • Commonly found on the leeward side of pan margins
  • Heights of 30 m relative to the pan floor
• The combination of vegetated orange–red dunes, dry valleys, and seasonal pans in the southern Kalahari creates a landscape of scientific and aesthetic significance
• Pans and valleys serve as sediment sources for dunes and also influence dune patterns
• Thick duricrusts affect the morphology of drainage systems and undergo chemical modification as water flows through them
• Kgalagadi Transfrontier Park protects a portion of the Kalahari, allowing for wildlife conservation and geotourism
• The Western Free State Panfield is a landscape of myriad pans and lunettes
• Pans


  • Enclosed depressions found in dryland areas and are widespread in southern Africa
• Lunette Dunes


  • Characterized by crescentic morphology
  • Often flank the downwind margins of pans
  • Composed of sediment derived from the pan floor and upwind sources
  • Sediments range in size from clay to sand, with clay attributed to panfloor deflation and sand to littoral sand moved by wave action
• Climate and Geology of the Free State Landscape

  • Located within the catchments of the Vaal and Orange Rivers
  • Drakensberg forming its eastern boundary
  • Experiences maximum rainfall in February and March due to the summer Indian Ocean monsoon
  • Winds are variable, primarily from the north to northeast, with northwesterly winds associated with cold fronts in late winter
  • Predominantly composed of late Paleozoic and Mesozoic sedimentary rocks of the Karoo Supergroup
• Sediments


  • Range in size from clay to sand
  • Clay attributed to panfloor deflation
  • Sand attributed to littoral sand moved by wave action